This invention relates to the production of laser radiation throughout the volume or bulk of a semiconducting material.
Conventional junction semiconductor lasers, such as the GaAlAs laser diode produced by Laser Diode, Inc., generate laser radiation with about 15 milliwatts average radiative power delivered at a peak intensity wavelength of around 850 nanometers (nm), with external quantum efficiency of about four percent; average power dissipated is around 375 milliwatts. Some of these performance limitations arise from the nature of the junction semiconductor laser itself, wherein the laser radiation is generated only in or adjacent to the thin, small volume depletion layer formed near the common boundary of the n-type and p-type materials that comprise the semiconductor.
A bulk semiconductor laser that produces radiation of an appropriate wavelength substantially throughout the bulk or volume of the semiconductor material would be an attractive alternative to the junction semiconductor laser and certain solid state and gas lasers as well. The average radiative power delivered by a bulk device could be at least four orders of magnitude higher than the average power delivered by the junction device, and the external quantum efficiency could be improved by a factor of about five.
In 1959, Basov, Vul and Popov predicted that bulk laser action in silicon and germanium could be promoted by applying sufficient voltage across the semiconductor material to cause avalanche breakdown. No experiments have ever verified this, because the rates of radiative recombination in silicon and germanium are relatively low compared to those of competing non-radiative processes.
In Jour. of Appl. Physics 38 (1968) 4589 Southgate reported what he believed was semiconductor laser operation in bulk, using the rather poor quality gallium arsenide material available at that time. Southgate applied a high voltage across the bulk semiconductor to achieve avalanche breakdown. The recombining electrons (from the avalanche conduction) emitted photons. Southgate's device had very low efficiency (less than 0.01 percent) and operated at an average electric field of 3,000 volts/cm within the bulk. A device operated at such field values will have difficulty attaining avalanche breakdown uniformly throughout the bulk, and self-absorption of photons generated within the bulk will dominate the radiative processes and reduce the efficiency by orders of magnitude.
In "Evidence for Avalanche Injection Laser in p-Type GaAs," Appl. Phys. Lett. 7 (1965) 225, Weiser and Woods reported the experimental observation of laser action induced by avalanche breakdown in Mn-doped p-type GaAs into which Zn, an acceptor, was diffused a short distance from each end of the GaAs block. Sharp voltage drops occurred across the Mn-Zn common boundary regions, which were each about 5 micrometers (.mu.m) thick Above a critical end-to-end voltage of 11-15 volts that depends upon ambient temperature (T=4.2-300.degree. K.), the device manifests negative resistance and radiation is promptly emitted at or around wavelengths .lambda.=840 nm and 890 nm, corresponding to electron recombination on a Zn center and a Mn center, respectively, at temperature T=77.degree. K. The corresponding current density at T=77.degree. K. was estimated at 10.sup.4 Amps/cm.sup.2. The authors note that the severe heating problem forces them to use voltage pulses of time duration 0.1 usec or shorter.
An optically pumped bulk semiconductor laser, with uniform pumping assumed throughout the bulk, was examined theoretically by Magee and Haug in I.E.E.E. Jour. of Quantum Electronics QE-6 (1970) 392-400. By directing a laser at the bulk semiconductor material, photogenerated charge carriers are produced. When these charge carriers recombine, they generate photons, which can give a lasing action with the proper structure. Their results predict a sharp rise, by two or more orders of magnitude, in power output from a thin platelet of solid GaAs for a small increase in pump intensity near threshold. This agrees in general with experimental results reported earlier by Basov, Grasyuk, Ehfimov and Kamulin in Soviet Physics-Solid State 9 (1967) 65-74, where a ruby laser was used to pump bulk GaAs maintained at liquid nitrogen temperatures.
Chinn, Rossi, Wolfe and Mooradian, in I.E.E.E. Jour. of Quantum Electronics QE-9 (1973) 294-300, report the pulsed operation of GaAs and Si-doped GaAs platelet lasers at wavelengths in the range 0.88 .mu.m&lt;.lambda.&lt;0.89 .mu.m, using optical pumping radiation from GaAs, GaAlAs and GaAsP laser diodes, from a Raman-shifted ruby laser, and from a two-photon (pumping) Nd:YAG laser. Pump frequencies were varied from 25-80 meV above the bandgap energy, and the output power abruptly increased by a factor of about ten as the input power was raised from 20 to 30 watts. Maximum power efficiencies obtained from GaAs and Si-doped GaAs were 3.3 percent and 6.3 percent, respectively.
In Applied Physics Letters 38 (1981) 507-509, Roxlo, Bebelaar and Salour report operation of a tunable bulk CdS platelet laser, optically pumped by an Ar+ laser at .lambda.=0.458, 0.473, 0.476 and 0.488 .mu.m. Pump thresholds as low as 25 mW (100 kW/cm.sup.2) were found, and the power conversion efficiency was 10 percent. Generation of picosecond pulses from bulk GaAs and CdS.sub.0.5 Se.sub.0.5 platelet lasers is discussed by Vaucher, Cao, Ling and Lee in I.E.E.E. Jour. of Quantum Electronics QE-18 (1982) 187-192. The laser was optically pumped by two-photon pulses of unspecified duration from a Nd:glass laser. This allowed a tuning range of 0.84-0.855 .mu.m for GaAs. At a pump intensity of 134 MW/cm.sup.2, the amplified spontaneous emission increased abruptly by three orders of magnitude and the bandwidth simultaneously narrowed to 0.002 .mu.m for GaAs. Operating efficiency is not specified.
Bulk semiconductor switches which use avalanche breakdown are described in U.S. Pat. No. 4,347,437 to Mourou and U.S. Pat. No. 4,438,331 to Davis. These patents show the triggering of avalanche conduction in a bulk semiconductor by directing a laser trigger at the bulk semiconductor to initiate avalanche conduction with a high voltage applied across the bulk.